Uranium-base alloys

ABSTRACT

Nuclear fuel uranium-base alloys are provided containing 3.5 to 3.7 wt. percent silicon and 0.1 to 1.5 wt. percent aluminum. On heat treatment, the structure consists mainly of a matrix of delta phase U3Si with a minor amount of UAl2. Improved processing properties are obtained over the binary delta U3Si alloy, and better corrosion resistance compared to a similar ternary alloy with less Si and a free uranium phase.

United States Patent [191 Wyatt, deceased et al. 1 Feb. 20, 1973 [54] OTHER PUBLICATIONS Inventors! Brian Sidney Wyatt, m late Metallurgical Structure in a High Uranium-Silicon Alof Ottawa, by Margaret I. Wyatt, administratix, Kanata, both of Ontario, Canada Atomic Energy of Canada Limited, Ottawa, Ontario, Canada Filed: June 8, 1970 Appl. No.: 44,611

Assignee:

Foreign Application Priority Data June 16, 1969 Canada ..054,506

References Cited FOREIGN PATENTS OR APPLICATIONS 6/1963 France .1... ..7s/|22.7

loy; Wyatt et al.; Atomic Energy of Canada, Ltd.,-

Chalk River-AECL-276l Primary Examiner-Carl D. Quarforth Assistant Examiner-R. E. Schafer Attorney-Alan A. Thomson 5 7 ABSTRACT Nuclear fuel uranium-base alloys are provided containing 3.5 to 3.7 wt. percent silicon and 0.1 to 1.5 wt. percent aluminum. On heat treatment, the structure consists mainly of a matrix of delta phase U Si with a minor amount of UAl,. Improved processing properties are obtained over the binary delta U Si alloy, and better corrosion resistance compared to a similar ter- 'nary alloy with less Si and a free uranium phase.

4 Claims, No Drawings URANIUM-BASE ALLOYS Delta phase uranium-silicon (U Si) alloy is potentially useful as nuclear reactor fuel material but is difficult to process or fabricate and is subject to swelling on irradiation. Design of the fuel element can minimize the swelling problem without sacrificing corrosion resistance, or increasing neutron absorption.

Previously U-(3.4 to 4.0 wt. percent) Si alloys if completely transformed to the delta phase U Si during heat treatment, have been found to have good aqueous corrosion resistance. However where large dendrites of the primary phase U Si were present in the as-cast structure, difficulty was experienced in achieving complete transformation to the delta phase. The presence of residual free uranium in the heat treated structure resulted in significantly poorer corrosion resistance. One ternary U-Si-Al was tested, containing insufficient Si to avoid leaving a free uranium phase, but after annealing corrosion resistance to high temperature (about 300C and above) water was much inferior to that of the equivalent completely transformed binary alloy.

I have now found that if the amount of Si in such ternary alloys containing small amounts (up to 1.5 wt. percent) of Al, is increased so that on annealing to form the delta phase no free uranium phase remains, the corrosion resistance to high temperature water and swelling resistance and other irradiation properties are at least as good as the delta U Si binary alloy. Also unexpectedly the time required at annealing temperature to effect the complete transformation to delta phase U Si when large dendrites of U Si, are present in the as-cast structure is decreased compared to the binary alloy.

Alloys within the scope of this invention have from 3.5 to 3.7 wt. percent silicon, from 0.1 to 1.5 wt. percent aluminum and balance uranium (except for impurities). With less silicon, free uranium remains and the high temperature water corrosion resistance suffers, while with more silicon an excess amount of the brittle dendrite U Si phase is formed which cannot be removed by subsequent heat treatment. Below 0.1 wt. percent Al, the improvement in fabrication and annealing behavior is negligible and above 1.5 wt. percent Al, the neutron economy, and corrosion resistance are poor. Preferred ranges are 3.53 to 3.58 wt. percent Si and 0.3 to 1.25 wt. percent Al. The total silicon plus aluminum should be sufficient to combine with all of the U metal phase during heat treatment.

The heat treatment of the alloys is necessary to optimize corrosion resistance and produce material of maximum density by forming a delta phase U Si matrix, a uniformly dispersed U-Al phase and at most only isolated fine particles of ms, The temperature should be at least about 800C but should not be above about 850C. The heat treatment is continued until substantially complete transformation of the U-Si to delta phase U Si has occurred (usually about 72 to 144 hours).

The following examples are illustrative.

EXAMPLE 1 Alloys were produced by induction melting in a high frequency furnace under a slight positive pressure of argon (to avoid degassing of the melt and the formation of large pores in the castings). The Si was added in the form of U Si bar and the Al as virgin metal. Zirconia crucibles were used in the furnace and the melt was cast into a graphite mold in the form of 15 mm diam. bars. The melt temperature was about 1,500C. The castings were then heat treated for 72 hours at 800C to form the delta U Si phase. The alloys were then corrosion tested in 300C water and typical results are shown in Table I.

The corrosion resistance of the 2.26 wt. percent Si alloy was very poor, but all of the higher Si alloys were comparable to the binary U Si alloy. The workability of these latter alloys was good and they could be extruded, cold-worked and otherwise fabricated without brittle fracture. Metallographic examination confirmed a matrix structure of delta U Si with no free uranium for the 3.5 3.7 wt. percent Si alloys. The 2.26 wt. percent Si alloy had large areas of untransformed free uranium in the structure after the heat treatment.

EXAMPLE 2 Response to heat treatment of the alloys containing 0.3 and 0.6 wt. percent Al from Ex. 1 was determined on re-casting in a vacuum furnace and slowly cooling to give large U Si dendrites approximately pm in width, together with a free U phase. A U-3.67 wt. percent Si binary alloy was also re-cast as a control. Samples from these casn'ngs were heat treated at 800C for 0.5, 1, 3, 5, 7, l0 and 22 days and the thickness of the U Si delta phase rim measured. Results indicated that the ternary alloys of the present invention transformed completely to the delta phase within 22 days whereas the binary alloy did not transform completely. Transformation of the ternary alloy was complete in 10-15 days.

EXAMPLE 3 A U-3.6 wt. percent Si 0.5 wt. percent Al ternary alloy and a U-4.0 wt. percent Si binary alloy were prepared and transformed to delta phase U Si as in Ex. 1. The alloys were then irradiated in a nuclear reactor for 60 full-power-days. There was no difference in behavior between the two alloys after a bum-up of 2000 MWd/T, showing that the ternary addition gives irradiation properties at least as good as the binary alloys.

Subsequent corrosion tests in steam at 550C with the alloy U3.6 wt. percent Si-0.5 wt. percent Al have shown that the corrosion rate of this alloy is about one tenth that of the binary alloy.

from 3.53 to 3.58 wt. percent.

3. The alloys of claim 1 wherein the aluminum content is from 0.3 to 1.25 wt. percent.

4. The alloys of claims 1, 2 and 3 wherein the structure consists of U Si, a minor amount of UAlg and isolated small particles of U Si i t i 

1. Uranium alloys consisting essentially of from 3.5 to 3.7 wt. percent silicon, from 0.1 to 1.5 wt. percent aluminum, and the balance uranium.
 2. The alloys of claim 1 wherein the silicon content is from 3.53 to 3.58 wt. percent.
 3. The alloys of claim 1 wherein the aluminum content is from 0.3 to 1.25 wt. percent. 